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ARTICLE
Received 00th January 20xx, Accepted 00th January 20xx DOI: 10.1039/x0xx00000x
Natural Deep Eutectic Solvents as an Efficient and
Reusable Active System for the Nazarov Cyclization
Stefano Nejrotti,a Marta Iannicelli,a Salwa Simona Jamil,a Davide Arnodo,a Marco Blangetti,a and
Cristina Prandi*a
Natural Deep Eutectic Solvents have emerged as alternative non-toxic, non-aqueous solvents for an increasing number of synthetic transformations. Remarkably, in some cases one (or more) components of the NaDES plays an active role in the reaction mechanism and directly participates as either a catalyst or as a reagent in the reaction. In this paper, we tested several NaDESs in which one of the components is a carboxylic acid as medium to perform Nazarov cyclizations on divinyl ketones to obtain cyclopentenones, a widespread motif in natural compounds. The reaction conditions were optimized and the scope investigated on C-, O- and N-derived compounds. To assess the full sustainability of the proposed approach, the recyclability and scalability of the process were investigated, thus proving that multi-grams preparations are possible with complete recycle of the medium.
1. Introduction
The Nazarov reaction or Nazarov cyclization (NC) was first reported in 1942 and consists in the 4 conrotatory electrocyclization of divinyl ketones to cyclopentenones in the presence of Brønsted or Lewis acids (Scheme 1).1 The reaction
is tolerant to substituents on the dienone skeleton (Scheme 1,
A), and thus allows for the facile synthesis of functionalized
cyclopentenones B, including relatively simple non-annulated structures as well as complex polynuclear cyclopentenoid skeletons annulated with one or more non-aromatic, aromatic or heteroaromatic rings.
Scheme 1. a. The Nazarov reaction. b. Examples of the occurrence of the cyclopentenone motif in natural compounds
During the years, the Nazarov reaction has found a wide application in synthesis and has become one of the most useful tools for the preparation of various cyclopentenone motifs.1b, 2
The study of the torquoselectivity of the reaction allowed for a stereochemical control of the substituted cyclopentenones
obtained,3 and enantioselective versions of the NC were
reported.4 Modern protocols for the facile generation of the
pentadienyl cation intermediate include a wide number of precursors beyond the traditional divinyl ketones A,5 including
oxiranes,6 unsaturated carbinols,7 allene ethers8 and
propargylic esters.9 Besides strong Brønsted acids, the NC can
be catalysed by Lewis acids, such as main-groups Lewis acids and transition metals: successful data are for example reported for BF3∙Et2O,10 Pd(0),11 Ag,12 Au(I),13 Au(III)9b, 14 and halides.15
Remarkably, organocatalytic versions of the NC were also reported in which chiral phosphoric acid derivatives,16
diamines,17 thioureas,18 cinchona alkaloids19 were also used.20
Despite its importance as a synthetic tool for the preparation of natural and pharmaceutically active compounds, and its undeniable potential as a synthetic tool which fulfills green chemistry principles in terms of atom economy, the NC still presents some drawbacks related to sustainability. Indeed, strong Brønsted acids (e.g. TFA, TfOH, MsOH), which are toxic and corrosive, are usually employed and are often required in a stoichiometric or super-stoichiometric amount.21 On the other
hand, the use of Lewis acids is affected by the cost and air-sensitivity of the metal catalyst. Furthermore, volatile organic solvents (VOCs), and in particular chlorinated solvents, are by far the most common solvent system for the NC.2e Up to now,
only few references report on more sustainable versions of the NC, in which non-toxic and recyclable homogeneous22 and
heterogeneous23 catalysts were used. A remarkable example in
this sense is represented by the cellulose-SO3H catalyst
reported by Rostami.24 Finally, a Nazarov cyclization performed
in water in the presence of surfactants (and Lewis acids) was reported.25
Deep eutectic solvents (DESs) have emerged as a new promising class of green solvents, since they are flammable, non-toxic, biodegradable and very easily prepared from cheap and a. Dipartimentodi Chimica, Università di Torino, via P. Giuria 7, I-10125
Torino, Italy.
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary information available should be included here]. See DOI: 10.1039/x0xx00000x
principles and are now foreseen as potential contributors to more sustainable industrial processes.28
Since the first report by Abbot,29 DESs have been successfully
employed in several fields of application,26c, 27 including organic
synthesis.30 It has been shown that, apart from constituting a
more sustainable alternative to traditional solvents, DESs often behave as non-innocent reaction medium and can have an active role in promoting organic reactions, by improving the reaction rate, the yield and allow for milder conditions. This has been attributed to the supramolecular structure of these mixtures, characterized by an extensive hydrogen bonding pattern.30c-e
In this work, we wish to report on our study on the Nazarov cyclization in deep eutectic solvents, open-air and under mild conditions. Naturally occurring carboxylic acids are employed as component of the DES, allowing for mild conditions in a renewable active solvent system, which combines the lower toxicity and good biodegradability31 with its low cost. As to our
knowledge, this is the first example of a NC promoted by weak carboxylic acids. Remarkably, the medium can be recycled up to 4 times without considerable loss in its activity.
1). At first, ChCl/L-(+)-lactic acid in 1:2 molar ratio was tested and revealed ineffective in promoting the reaction at room temperature, even after 24 h (Table 1, entry 1). On the other hand, ChCl/oxalic acid 1:1 partially converted the substrate at the same temperature (entry 2). Raising the temperature up to 60 °C resulted in an improvement of the reaction yield to 91% after 16 h (entry 3). NaDESs based on L-(-)-malic acid (entry 4), citric acid (entry 5) and L-(+)-tartaric acid (entry 6) were less effective, as lower yields were obtained with these NaDESs under the same experimental conditions. Conversely, malonic acid (entry 7) and maleic acid (entry 8) based NaDESs performed well, affording 2a with a yield of 92% and 87% respectively. ChCl/p-toluenesulfonic acid (TsOH) in 1:2 molar ratio was also tested, and in this case a yield up to 99% was obtained (entry 9).21, 31 NaDESs based on betaine (entry 11) and L-proline (entry
12), instead of ChCl, were found to be ineffective in promoting the reaction. We then performed some control experiments, which are summarized in Table 1, entries 13-16. A DES not containing an acid, such as ChCl/H2O 1:2 (entry 13), or a polar
organic solvent, such as ethanol (entry 14), were ineffective in promoting the reaction, indicating that an acid-containing DES is required.
Table 1. Study of the reaction conditions for the Nazarov cyclization of substrate 1a in NaDESs.
entry solvent T (°C) time 2a (%)a cis/transb
1 ChCl/L-(+)-lactic acid 1:2 rt 24 h traces -
2 ChCl/oxalic acid dihydrate 1:1 rt 16 h 30% 57/43
3 ChCl/oxalic acid dihydrate 1:1 60 16 h 91% 14/86
4 ChCl/L-(-)-malic acid/H2O 1:1:2 60 16 h 22% 26/74d 5 ChCl/citric acid 1:1 + 2% H2O 60 16 h 36% 67/33 6 ChCl/L-(+)-tartaric acid 1:1 + 2% H2O 80c 16 h 67% 17/83d 7 ChCl/malonic acid 1:1 60 16 h 92% 17/83 8 ChCl/maleic acid 1:1 60 16 h 87% 16/84 9 ChCl/TsOH monohydrate 1:2 60 16 h 99% 15/85 10 ChCl/R-(-)-mandelic acid 1:1 60 16 h 59% 20/80d 11 betaine/malonic acid 1:2 60 16 h - -
12 L-proline/oxalic acid dihydrate 1:1 60 16 h - -
13 ChCl/H2O 1:2 60 16 h - -
14 EtOH 60 16 h - -
15 EtOH (10 eq malonic acid) 60 16 h 33% 24/76
16 DCE (10 eq malonic acid) 60 16 h traces -
17 H2O (10 eq malonic acid) 60 16 h 23% 30/70
Reaction conditions: 1a (0.2 mmol), in 1 g of the appropriate DES. a Yield of isolated product. b Determined by 1H-NMR analysis of purified compounds. c Higher
ARTICLE
The acidic component of the eutectic mixture, malonic acid, was tested in either polar and apolar solvents such as ethanol (entry 15), 1,2-dichloroethane (entry 16) and water (entry 17), but very low yields were obtained in these cases, suggesting that the activity is a peculiar property of the eutectic mixture, and does not derive from its acidic component per se.33 Considering
that in general the acidity of the DES is governed by the acidity of the HBD, we then tried to rationalize these results, taking into consideration the pKa of the acidic component of the NaDES. The NaDESs containing malic acid (pKa 3.40), citric acid (2.92, 4.28 and 5.21) and tartaric acid (2.99) were almost ineffective, while the best results were obtained with oxalic (1.23), malonic (2.83) and maleic acid (1.93),34 with malonic acid better
performing (see entry 7, Table 1) than maleic (entry 8, Table 1). Taken together, these data do not account for a direct correlation between performance in the NC and pKa of the acidic component of the DES. According to a recently published
study,35 the acidic properties of DES mixtures can differ from
those of their components alone, and moreover they can be significantly affected by the temperature. This behaviour suggests the crucial role played by the supramolecular interactions in the eutectic mixture and is particularly relevant in the case of ChCl/oxalic acid 1:1 and ChCl/malonic acid 1:1, where a drop in the pH values are observed as the temperature increases.36 These observations are in line with our results,
which showed how in ChCl/oxalic acid 1:1 and ChCl/malonic acid 1:1 (entries 3 and 7 of Table 1) at 60 °C 2a was obtained in both case in 92% yield despite the different pKa of the HBD component (1.23 for oxalic acid and 2.83 for malonic acid). The diasteroselectivity of the reaction was also evaluated in terms of cis/trans ratio. At room temperature (entry 2) the cyclization is not diastereoselective. At 60 °C the selectivity improves in favour of trans-2a to a 14/86 ratio in the case of ChCl/oxalic acid (entry 3).
Similar results have been obtained with ChCl/malonic acid and ChCl/maleic acid (entries 7 and 8) thus proving that the most efficient NaDES are also those inducing the best diasteroselectivity. It should be noted that in the case of ChCl/citric acid (entry 5) the diasteroselectivity is in favour of
cis-2a. A proposed mechanism to account for the
diastereoselectivity either in favour of the cis or the trans cyclopentenone and based on the acidic character of the melt might involve multiple key activation roles of the solvent (Brønsted acid and hydrogen bond formation), as well as a plausible structure of the eutectic mixture. Further computational and physicochemical investigations are needed to elucidate these aspects. Regarding the enantioselectivity of the reaction, when we employed NaDESs composed of a chiral carboxylic acid (entries 4, 6 and 10) we did not observe any induction of chirality in the product.
Based on the optimization study, we extended the scope of the reaction to different substrates, choosing ChCl/malonic acid, ChCl/oxalic acid or ChCl/TsOH as possible reaction media (entries 7, 3 and 9 of Table 1). The results reported in Scheme 2 refer to the best score we obtained for each of the substrates. Symmetrical dienones bearing electron-rich or electron-poor aryl substituents were successfully cyclized to the corresponding cyclopentenones 2b-e with very good yields (Scheme 2). Sterically encumbered substrates leading to 2d and
2e afforded the corresponding products with almost complete
diastereoselectivity for the trans isomer.
Figure 1. Recyclability of the ChCl/malonic acid DES for the Nazarov cyclization of 1a. Yields were determined by quantitative 1H NMR analysis using CH
3NO2 as
internal standard.
Since one of the most synthetically useful aspects of the NC is represented by the possibility to access fused (hetero)polycyclic
molecular scaffolds, and due to our own research interest for the synthesis of heterocyclic natural compounds,37 we
investigated the reactivity of several dienones whose structure is embedded in an indole or dihydropyran moiety. With our delight, for all the substrates the reaction proceeded smoothly to afford cyclopentenones 2f-o in 78% to 95% yields. Finally, we showed that also aryl vinyl ketones, which are less prone to undergo the cyclization due to the interruption of the aromatic system as a consequence of the cyclization,38 are converted into
the corresponding Nazarov products 2p and 2q in good yield and diasteroselectivity.
One of the most attractive features of a catalytic system is undoubtedly the recyclability. We tested the possibility to recycle the ChCl/malonic acid eutectic mixture for the cyclization of dienone 1a, at a concentration of 0.2 mmol/g (i.e. mmol of substrate per g of NaDES). The procedure for the recovery of the NaDES, shown in Figure 3, is highly feasible and involves the addition of water to the reaction mixture upon completion of the reaction, causing the precipitation of the product, which can be collected by filtration. Water is then evaporated to restore the eutectic mixture, which is used without further manipulations for another run. As shown in Figure 1, the activity of the NaDES remains essentially unchanged for the first two cycles and is acceptable up to the fourth one, while in the fifth run a drop in the yield of product
2a is observed.
The activity of the NaDES strongly depends on the dynamic behaviour of its molecules and possibly in this case on the intermolecular interactions in which the COOH of the NaDES is involved. It seems then reasonable that the protic nature of the carboxylic acid component becomes stronger or weaker either in a more restricted or more disordered supramolecular network. To investigate how the activity of the NaDES changes at each recycling cycle we decided to evaluate the structure of the NaDES ChCl/malonic acid by 1H and 13C NMR (see SI for full
size spectra).39
Figure 2. 1H NMR of NaDES ChCl/malonic acid after recycling. a. ChCl/malonic acid
(*ChCl; °malonic acid). b. ChCl/malonic acid with 10% water w/w. c. ChCl/malonic acid after the first (1), second (2), third (3), fourth (4) and fifth (5) cycle.
Journal Name ARTICLE
This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 5
In Figure 2 the 1H NMR spectra of pure NaDES ChCl/malonic acid
are reported. The 1H NMR signals at 3.1 (9H), 3.4 (2H) and 4.1
ppm (2H) are attributable to ChCl (*), while at 3.5 ppm the CH2
of malonic acid is detectable (°). The COOH proton of malonic acid is entrapped in the tight network of intermolecular H bonds and is detectable in the 1H NMR spectrum as a very broad signal
between 10 and 11 ppm. In Figure 2, b, the 1H NMR of
ChCl/malonic acid in which a 10% of water has been added on purpose is reported. A signal at around 7 ppm is detectable and assigned to water entrapped in the NaDES, while free water is detectable at 4.7 ppm. In Figure 2, c, the 1H NMR spectra of the
NaDES recovered after each cycle are reported. It is evident that the structure of the NaDES remains substantially unchanged after the first cycle (Figure 2, c, 1), compared to the freshly prepared one (Figure 2, c, 0). In these cases yields of 2a of 94% and 97% respectively were obtained (see Figure 1). Starting from the NaDES recovered from the second cycle, a broad signal around 8 ppm starts to appear and becomes progressively more evident after the fourth and fifth cycle, shifting towards 7 ppm. The presence of some other minor peaks might indicate that the structure of the DES is partially decomposed.40 To rationalize
these results, it should be worthwhile to recall that the activity of the NaDES ChCl/malonic acid is higher than that of malonic acid alone (see entries 13-15 in Table 1), as a confirm that the supramolecular structure of the DES might enhance the polarizability of COOH groups and facilitate the reactivity. The increase in the amount of water contained in the DES (and not removable by simple evaporation) is apparently correlated with the observed drop in yield. Further investigations are needed to assess how the supramolecular structure of the NaDES is modified at each cycle.41
Figure 3. Two-cycles, gram-scale reaction.
The successful recycling procedure prompted us to further increase the efficiency of the process, with a view to its scalability. To this aim, we tested various substrate concentrations in NaDES, in order to find the highest concentration at which the yield of product yield remains unaltered, which was found to be 1 mmol/g, provided that the substrate was well dispersed in the medium and the stirring of
the mixture was effective. Therefore, we set up a gram-scale reaction starting from 1.05 g (4.0 mmol) of substrate 1a and 4.0 g of ChCl/malonic acid (Figure 3). The NaDES was recovered and used again for a second run through the above described procedure. To minimize waste at this scale preparation, the water recovered at the rotavapor after the first cycle was used again for the second cycle.
The overall process afforded 2.01 g of cyclopentenone 2a by using only 4.0 g of ChCl/malonic acid 1:1. The E-factor parameter for the process is 19.5, a value that confirms the green chemistry credentials of our protocol (see SI for the calculations). 42
3. Conclusions
In this paper, for the first time we have disclosed a conceptually new, flexible, versatile, and high-yielding Nazarov cyclization of divinyl ketones performed in deep eutectic solvents, in which the reaction medium acts in a dual role both as a solvent and as a promoter of the cyclization. The combination of divinyl ketones and NaDESs composed by carboxylic acids as an environmentally benign reaction medium enables access to the targeted compounds with excellent yields and good stereocontrol, at moderate temperature. The scope of the reaction is wide and several substituents on the divinyl ketone framework are well tolerated.
Furthermore, the potential of this reaction in terms of Green Chemistry principles coherence was addressed and several sustainable features of this transformation have been developed. In particular, we demonstrated that the process is scalable and multi-gram preparations are accessible. A practical and easy work-up allows for the recovery of both the NaDES and water. We demonstrated that 4 cycles of reaction are tolerated with an overall acceptable yield. The beneficial effect of the DES is further showcased by its ability to obtain, in some cases, pure
compounds, thus avoiding purification by column
chromatography.
4. Experimental section
4.1 Materials. Flasks and all equipment used for the generation and
reaction of moisture-sensitive compounds were dried by electric heat gun under nitrogen. Unless specified, all reagents were used as received without further purifications. Anhydrous THF was obtained by distillation over LiAlH4, followed by distillation over
Na-benzophenone; benzaldehyde was distilled under vacuum. Flash column chromatography was performed over silica gel (40-63 μm, 230-400 mesh); Rf values refer to TLC carried out on silica gel plates.
1H NMR and 13C NMR spectra were recorded on a Jeol ECZR600, in
CDCl3, using residual solvent peak as an internal standard (CHCl3, 1H:
7.26 ppm, 13C: 77.16 ppm). NMR spectra of DESs were recorded in a
capillary tube, using D2O as locking solvent. Multiplicities are
reported as follows: s (singlet), d (doublet), t (triplet), q (quartet), quin (quintet), sext (sextet), m (multiplet), br (broad). GC-MS spectra were recorded at an ionizing voltage of 70 eV. Deep eutectic solvents were prepared by methods reported in the literature (see SI for
layers were dried over anhydrous Na2SO4, filtered and the solvent
was evaporated under reduced pressure. When required, the crude product was purified by flash column chromatography.
4.3 General procedure for recycling cycles. A vial was charged with
the dienone 1a (1.0-4.0 mmol), then ChCl/malonic acid 1:1 (5 g or 1 g per mmol of substrate) was added and the mixture was stirred at 60 °C for 16 h. Water was added, causing the precipitation of the product. The solid product was recovered by filtration, while water was evaporated under reduced pressure to afford the eutectic mixture, which was used for the next reaction cycle.
2,5-dimethyl-3,4-diphenylcyclopent-2-en-1-one (2a)24
Synthesized according to the general procedure with ChCl/malonic acid 1:1. White solid, 92% yield,
trans/cis ratio 83:17. Rf 0.25 (PE/Et2O 9:1). trans-2a
1H NMR (600 MHz) δ (ppm): 7.34-7.24 (m, 5H), 7.23-7.19 (m, 2H), 7.15-7.11 (m, 1H), 7.09-7.06 (m, 2H), 3.98 (quin, 1H, J = 2.2 Hz), 2.41 (qd, 1H, J = 7.3, 2.8 Hz), 2.03 (d, 3H, J = 2.0 Hz), 1.36 (d, 3H, J = 7.5 Hz). 13C NMR (150 MHz) δ (ppm): 211.1 (Cq), 167.2 (Cq), 142.2 (Cq), 136.9 (Cq), 135.3 (Cq), 129.1 (CH), 128.9 (CH), 128.5 (CH), 128.4 (CH), 127.7 (CH), 126.8 (CH), 56.5 (CH), 51.4 (CH), 15.4 (CH3), 10.3 (CH3). GC-MS m/z (%): 262 [M]+ (100), 247 (88), 219 (38), 115 (36). cis-2a 1H NMR (600 MHz) δ (ppm): 7.40-7.37 (m, 2H), 7.34-7.24 (m, 4H), 7.24-7.19 (m, 2H), 7.18-7.16 (m, 1H), 7.00 (br s, 1H), 4.63-4.60 (m, 1H), 2.92 (quin, 1H, J = 7.4 Hz), 2.09 (d, 3H, J = 1.7 Hz), 0.76 (d, 3H, J = 7.5 Hz). 13C NMR (150 MHz) δ (ppm): 211.6 (Cq), 166.4 (Cq), 139.3 (Cq), 137.1 (Cq), 135.9 (Cq), 129.1 (CH), 128.5 (CH), 128.5 (CH), 128.4 (CH), 127.7 (CH), 127.0 (CH), 52.7 (CH), 45.6 (CH), 12.4 (CH3), 10.4 (CH3). GC-MS m/z (%): 262 (100), 247 (92), 219 (40), 115 (40).
Conflicts of interest
There are no conflicts to declare
Acknowledgements
The authors acknowledge Prof. Vito Capriati for helpful discussions and advice, Dr. Matteo Tiecco for providing betaine/malonic acid DES, Huvepharma srl, CRT (Cassa di Risparmio di Torino), Regione Piemonte and the Italian Ministry of Research for financial support.
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